We have exact same problem in our '85 Westy.  We too have a replacement
(Panasonic) radio/tape player installed by the PO.  As you say, on AM when
the heater fan is on you get static but what even more strange is if we
turn on our headlights ( we always drive with them on in the day too) it
stops the static.  Go figure!  I'm interested in seeing what the "fix" is
for this.

Wendy & John
'85 Westy,   "S-CAPE"